ASTR 220 - Collisions in Space
Drake Equation Discussion Results

In an attempt to estimate the number of civilizations in the galaxy which
might be capable of interstellar communication (e.g., with radio), astronomers
often use Drake's equation. The Drake equation is a list of probabilities
which result in the estimate of the number of stars with planets harboring
such civilizations given the number of stars in the galaxy:

N = R**fp*ne*fl*fi*
fc*L

Here
N = the number of communication capable civilizations
R* = the formation rate (number per year) of stars suitable for
supporting life
fp = the fraction of those stars with planets
ne = the number of planets per star suitable for life
fl = the fraction of those planets which develop life
fi = the fraction of planets with life which develop intelligent
life
fc = the fraction of intelligent species which develop technology
(like radio telescopes) capable of interstellar communication (or at least
of broadcasting "we exist!")
L = the average lifetime (in years) of such a technologically advanced
civilization

Below is a table of the responses of the various discussion groups on
Apr. 18, 2000:

Obviously, our estimates of the number of intelligent species currently residing
in our galaxy with technological capacity comparable to or greater than our
own varies wildly. That is simply because even our best guesses for each of
the pieces in Drake's equation are still just guesses. A number larger than
one indicates the number of civilizations currently estimated to exist in the
galaxy among its hundreds of billions of stars. The most optimistic number
we came up with today is 5, which is still only 1 per hundred billion
stars - the proverbial needle in a haystack. The pessimistic numbers
(those less than one) suggest that on average, there is no such civilization
present in the galaxy. We qualify as one, so this implies that our
civilization is a rare and transient phenomenon.

I find it progressively more difficult to estimate each term in Drake's
equation. The fraction of intelligent species which develop technology
and the lifetime of their civilizations I find particularly difficult.
Even if a species is "intelligent," it is far from obvious to me that it
is likely to develop the sort of technology necessary to send a signal
across interstellar space. And the lifetime - well, I suspect that has
an odd distribution. I suspect most species are likely to destroy themselves
within a few generations of being able to do so (or at least knock themselves
back to the stone age). If one is able to survive for dozens of generations,
then it might find a stable state capable of lasting a very long time
indeed. The "average" lifetime of such a distribution is not tremendously
meaningful.

ET

If we are interested in the possibility that we have been visited by aliens,
we can modify the Drake equation to estimate the probability that this might
happen. To do this, we simply multiply the above equation by two new
terms:
ft = the fraction of technological civilizations which develop
the ability to travel between stars
pe = the probability that such an interstellar-mobile species
has visited Earth
The modified Drake equation now becomes:

N = R**fp*ne*fl*fi*
fc*L*ft*pe

These new terms are difficult to estimate. The most likely answer for
ft is binary: either zero (it is physically impossible) or
one (those who make it this far in the Drake equation eventually figure
out a way). The probability of visiting the Earth is low - we are just
one planet out of 10 in our solar system, and our star just one of hundreds
of billions. There are 6 billion humans right now; even if we each had
our own personal star ship, we would each have to personally scour many
dozens of star systems to have visited the entire galaxy. Of course, a
civilization which endures a long time has a better chance of eventually
coming here, so these issues are hard to separate. Nevertheless, I would
think it very optimistic to adopt
ft = 1
pe = 10-6
Even our most optimistic estimate makes the prospects of meeting ET quite
dim, especially within our own brief lifetimes.